Still Learning! Ouch!!

[@calimark7448]

I just got a Pegasus Astro Falcon rotator. I love it. Also no real back focus problem on the Askar FRA400. I just got the 294mm Pro, yes it has it's problems and I would have gone for the 2600mm but the extra $$ weren't there at the time, Now that I'm using it I do like it even with the amp glow and the bin1 mode is awesome. I miss the wider field of view from the 2600 though. I consider my rig complete now FRA400 w/.7 reducer (285mm f3.9), AM5, 294mm Pro, 7 filter wheel w/Antlia 36mm 3nm filters, ZWO EAF, Falcon 2 rotator, Giotto and Alto, Pegasus Power Box, Mele Quieter 3. I just got an EdgeHD 9.25 and getting that set up has been... a pain. I love the images though. Clear skies!

[@AstroVagabond]

Wow, nice setup on your FRA400! Keep me posted on your EdgeHD 9.25. Seems like a potential upgrade to my EdgeHD 8 at some point down the road.

[@robinbrown3896]

Bill, Thank you for your reply about the 'Focuser Movement' of 23mm. Now the claim is that the REDCAT51 does not have a back focus, this is sort of true, but not entirely correct. We need to understand how the Petzval telescope works when it comes to focusing. The fixed lens in the telescope will move as a complete unit of 23mm in total when moving the focuser. The 55mm back focus needs to be taken into consideration and should be adhered to when adding equipment to the front of the telescope. For instance if you just connected the ASi camera to the front of the telescope you will not be able to get focus. Spacers will have to be used to move the camera back far enough to get focus. You mentioned the telescope focuser has about 23mm of movement. So if we added just an ASI camera and the appropriate spacers to add up to 55mm, then moved the focuser to position 11.5mm from its zero position, the camera will be in focus, at this position. There will be 11.5mm each side of the focal plain. This meaning you have 11.5 in front and 11.5mm behind the camera sensor to get that perfect focus. Now, when it comes to 'The REDCAT51 does not have back focus' Which it does, it just means there is some free space in front and behind that allows the user to be slightly out with 55mm back focus and still get focus. If for instance another spacer 10mm was added to get a value of 65mm back focus, then technically we would be able to get perfect focus in absolutely perfect conditions which don't exist, we would have 1.5mm to try and achieve focus. So if we added an extra 5mm adding to the 55mm back focus to 60mm then we would still be able to get focus having 5.5mm left to achieve it. This would also apply if we only had 50mm and not the 55mm back focus. The 55mm back focus needs to be taken into consideration and should be adhered to if possible when adding equipment to the front of the telescope. So I did some calculations to see if it is possible to add the rotator, and this is what I came up with. You mentioned that the Back Focus is 55mm, so this is a good start and all we have to play with. Camera ASI2600 has a 12.5mm Tilt adapter removed. Filter Wheel 20mm Attached to the camera. Rotator 11.8mm Note: this is the correct value as the body is 10mm, but the center wheel protrudes by 1.8mm giving 11.8mm. Total ----------------- 44.3mm So far we are well in the ball part of 55mm. Spacer/Adapter #1 5mm This is to connect the Rotator to the filter wheel. Spacer/Adapter #2 6mm This is to connect the Rotator to the telescope. Total ----------------- 55mm Bingo you are right on target of 55mm. Well, 55.3mm, close enough. If one wants to take into consideration filter offset, then we need to add 0.5mm. So, Spacer/Adapter #2 would be need to be 6.5mm OR, this can be added to Spacer/Adapter #1 making it 5.5mm Total ----------------- 55.8mm Which will work just fine. So when you move the focuser in or out, all that is happening is you are are moving the distance to the camera sensor back to 55mm to get focus. This is where 55mm back-focus comes from with the Petzval design. Something for you to think about, if you ever wanted to go in this direction connecting the Rotator. If you ever wanted to have customized adapters made then goto the following link. I have had all my Adapter/Spacers made up there. They are reliable and fast. rafcamera.com/ All the best for your remote site new astro adventure.

[@AstroVagabond]

Hi Robin, thank you once again for the detailed information. Very helpful. For now I'm going to deploy without the rotator. I going to keep it simple and see if I run into targets where I feel I really need the rotator. Much appreciated and I'll capture your comments above in a document for future consideration. ~ Bill

[@robinbrown3896]

Bill, what is the max amount of movement in mm does the focuser have. Using the scale on the side of the telescope.

[@AstroVagabond]

Hi Robin, I think the max amount of the helical focuser movement is effectively 22mm. The RedCat has what's called a working distance of around 55mm.

[@OldGirlPhotography]

I've never used a rotator and use both of the cameras you mention. Logically, not sure why you would need a rotator on a wide field scope. Also, I shoot my calibration frames once per season because my field of view orientation does not change. This gives me a great result. If you had a rotator, you would need to shoot calibration frames each time you used the scope. Way too much work - at least for me.

[@AstroVagabond]

Great points! I think the decision to purchase was a carryover from my days as a photographer where I was known to suffer from GAS (gear aquisition syndrome). 😉 I like to learn about new things and wanted the experience of using a rotator. That's why I had no hesitation to remove it. Thank you for taking a moment to drop a comment. Much appreciated. 👍

[@Diocrew]

Hi Bill, just curious, are you only moving the Redcat down to Starfront?

[@AstroVagabond]

I'm moving the RedCat and the EdgeHD 8 as well. Dropping both off at Starfront on December 29th. I expect it will take them a couple of weeks to put in place on the piers before I can remote intothem. I expect I will learn a lot! 😉

[@Diocrew]

Oh awesome!

[@JonnyBravo0311]

Let's talk calibration. Your light frame, i.e. the one you take under the stars of whatever target you're shooting, contains a number of unwanted things. Sensor noise. Optical defects. Dark current. Calibration removes those unwanted things. The darks remove additive things like dark current / amp glow. The flats remove multiplicative things like light falloff. But here's the thing - those flats are just lights, so they contain the same type of additive issues like dark current and amp glow. Just like you subtract a dark from a light, so too do you subtract a dark from a flat. Wait a second... so then what is this bias and why do people use them to calibrate a flat? Well, a bias is just an extremely short dark, typically taken at the shortest exposure your camera can do. For cameras like the 294 or 2600, that is 32 microseconds. So why do people use a bias in lieu of a dark to calibrate a flat? Simple: it's convenient. Flats (which remember are just lights) are usually pretty short exposures. We're talking a few tenths of a second or less, especially if you're taking sky flats using our own sun as your light source. Because it is so short, there really isn't time for all of those additive things to have any real impact on the data. The only additive thing we really need to remove is the offset, which is precisely what the bias frame represents. Thus, calibrating a flat with a bias is "good enough". In most cases. The sensor in the 294 is quirky. Whereas quite a few other cameras behave in a predictable, easily modeled linear fashion, the 294 doesn't at extremely short exposures. In fact, it's not until you've gotten to exposure times of around 2 to 3 seconds that the camera behaves properly. This means there's plenty of time for those nasty additive elements to play havoc, and thus you must take matching darks to subtract them all out. Now, there are plenty of people who will swear up, down, left and center that they can take short flats and use biases with their 294. Maybe they got a good sample of the sensor. Maybe they're not getting the best calibration they can and deal with it during image processing. Maybe they just don't know. There are hundreds (perhaps even thousands) of pages of debate on the astrophotography fora about using darks or biases to calibrate flats. My stance on the matter is that using matching darks with your flats is the best way to calibrate, regardless of which sensor you're using. Having written that, with my Poseidon-M Pro (same IMX571 sensor in the 2600MM Pro), I use biases to calibrate my flats and don't even bother taking darks for my lights, instead using the same biases to calibrate them. The old adage do as I say, not as I do is certainly apropos :).

[@AstroVagabond]

Love it! Thanks Jonny for your willingness to share your knowledge. Need to do an interview with you one of these days. 😉