iantaylor2uk

You should have made it clear you only wanted advice that you agree with!

Yes, agree. In fact the telescope at the bottom of the large refractor was described to me as an astrograph, and you can see the wooden end where photographic plates were inserted.

I think most experienced astrophotographers would consider "get the best mount you can afford" very good advice. I personally don't think a mono camera with narrowband filters is a good starting point for a beginner. You have to get used to focussing, which is more difficult with narrowband filters, and you will need longer exposure lengths, which will require a reasonable mount. A cooled OSC camera would be a better starting point in my opinion, but you still need a good mount.

I was fortunate enough to be shown around the University of Central Lancashire's Alston Observatory by Mark Norris. They have a Planewave 700 CDK telescope and they also have a 15" refractor from the 1920s which is not currently in use as the dome contains asbestos. Mark also told me they are hoping to get a Planewave Delta Rho 350 (they already have a Planewave 350 mount). Unfortunately when I was there it was cloudy but still a very impressive facility.

Photons don't have mass but they do have energy and momentum. A photon can lose energy by "falling" in a gravitational field, and will become red shifted.

You have to be careful applying Newton's laws of motion to photons - if you say the force is proportional to the change of momentum you are OK (as photons have momentum) but you can't say force is mass times acceleration, since the mass of a photon is zero, and photons can only accelerate by changing direction (as their speed is constant). All this assumes the photon is in a vacuum. You can also use conservation of energy laws too, but clearly the standard equation for kinetic energy (as being equal to half*mass*speed squared) doesn't hold.

I would encourage you to play with the gain value - a lot of people stick with low gain or unity gain as they think it is better to keep a higher dynamic range, but many people have had good results with using higher gain values, and you also get lower noise at higher gains. You also get some of the dynamic range back by stacking a few hundred subs.

What gain are you using? With no filter I usually use 30-60 second subs, and only go up to 2 mins if I use an L-enhance filter, but I tend to use pretty high gain (200 on my ZWO 071 which has a max gain of 240). Most people with CMOS cameras tend to use shorter subs (as compared to CCDs) and take many of them.

I have found the L-enhance filter is very good but you will need longer exposures than with no filter.

Electromagnetic waves travel at the speed of light (in a vacuum) but the electrons flowing through a wire will travel much more slowly

I would say a HEQ-5 is a good starter mount if you are imaging in the 500-800mm focal length range. If you are happy with shorter focal lengths (135-300 mm) to start with, I would advise to go with a tracker mount, and get used to astrophotography and see if you like it before taking a deeper dive in.

Although the advice that guide RMS in arcseconds must be half or less that of the imaging scale is often stated, I think it is a bit too conservative. My reasoning is as follows: Let's say guide RMS is 1" - then that would suggest that you need to be imaging at 2" or greater. However, to get a guide RMS of 1", if RA and DEC errors are equal, then the RA and DEC RMS would only be around 0.71", so I would have thought you would still be OK imaging down to a main camera scale of 1.4". Also, all of this ignores the effects of seeing, which could easily be 2-3".

I use two cheap 5 volt USB CooWoo dew heaters from Amazon - one for the guide scope and one for the main scope (102mm refractor). I plug one of the USBs into my mount and the other into a spare USB hub on the back of the camera. This works well for me.

I use a ZWO 071 camera and I think it is RGGB. The images come out quite green but there is an auto colour balance feature which sorts this out.

I have a G11 which I bought new in 2009 and I can see it lasting until another 30 years. It's pretty easy to clean, lubricate and take apart, and spares are readily available. The gemini-2 goto system is pretty easy to use and there is great technical support from Losmandy. On the other hand it is large and heavy. It is quite old technology and to be honest if I was buying now I would be tempted by the harmonic drive mounts.

I use Nebulosity 4 which I think is now free. That is good for basic processing after DSS, and then I may use Affinity for finishing up. Gradient removal in Siril sounds interesting, will have to look into that.

Sorry I was saying both the L-extreme and L-enhance filters need longer subs than with no filter or UV/IR. I usually use 2 minute subs for the L-enhance and 30-60 secs with a UV/IR filter, although I use fairly high gain on my camera.

3 mins is ok for the L-enhance, but you need to divide the exposure time by 3 to get the right exposure time for no filter or a simple UV/IR filter, so I think a better comparison would have been comparing against a stack of three one minute exposures.

Well possibly. The WO 81 GT IV is f5.9 whereas the Tak is f/8, so it may be just a case of optimizing the flattener spacing for each scope. The longer focal length of the Tak will help too

When I use the same flattener on my Takahashi 102 TSA f/8 refractor the field is noticeably flatter than for the WO 81 GT IV certainly.

Here's a jpg of a single fits image I took of M42 on 15th December with the WO 81 GT IV + 1x Hotech flattener using an APS-C sized sensor. I uploaded the image to astrometry.net and it told me the FOV was 2.85 degrees x 1.89 degrees, and if I put the WO 81 GT IV + 1x flattener + ZWO 071 camera into the online field of view calculator (https://astronomy.tools/calculators/field_of_view/) it tells me it should be: 2.83 degrees x 1.88 degrees, so the flattener may be very slightly less than 1x but is certainly pretty close. It's possible from pixel peeking at the corners that I may need to increase the spacing on the flattener slightly - however you should see what it looks like if you don't use any kind of flattener at all!

It is thermal shot noise (not read noise) that is reduced by cooling the sensor - I'm pretty sure I've seen a graph from ZWO which shows how thermal noise is reduced by cooling the sensor. Also be aware in the adverts that the lowest read noise is at the highest gain, whereas the highest dynamic range is at the lowest gain, so you can't get both simultaneously on your camera. I think you get diminishing returns past -5C, so I tend to use that value (you will still get lower thermal noise by going colder, but this is at the expense of more cooling power to the sensor and more chance of ice issues on the sensor).

I think it's unlikely that it would work with a full frame, but it definitely seems flat with an APS-C sensor provided you tune the spacing correctly (you need 55 mm spacing from shoulder of flattener to camera sensor, so for my ZWO 071 MC Pro camera (which is 17.5 mm in) you need 37.5 mm of spacers, and I it comes with some spacers, so you don't need many extra. There is a photo of my set-up below - I'll take a close-up view when I'm next out with the scope. There is a good YouTube video of the flattener (which works for any refractor between f/5 and f/8) at:

I use a William Optics 81 GT IV f5.9 refractor with a 1x Hotech flattener which gives a focal length of 478 mm and has a good flat field with the APS-C sensor on my ZWO 071 MC Pro camera.

Looks good, I recently got a 2nd hand TSA 102 which is excellent, so I'm sure the 120 will be even better.